In this type of signal processing circuit, suppression of carrier leak and prevention of degradation in modulation accuracy in a quadrature modulator constitute essential performance for ensuring communication quality and for observing the law.
Here, the carrier leak corresponds to a DC offset (DC offset) of I/Q-components in terms of an input of a quadrature modulator. Factors which affect the modulation accuracy of the quadrature modulator include the carrier leak, variations in level between I/Q-components (a factor of an I/Q mismatch, later described) in terms of an input of the quadrature modulator, and the quadrature nature of a local signal applied to the quadrature modulator, i.e., variations in amplitude and phase difference between the I/Q-components (a factor of the I/Q mismatch, later described).
In recent years, however, although a transmitter is required to provide higher performance in connection with increasingly faster communications, the above-mentioned performance rather tends to degrade due to constraints on circuit configuration resulting from the transition of the architecture toward direct conversion, and a reduction in voltages of the processes.
Accordingly, a signal processing circuit is essentially provided with means for compensating a quadrature modulator for a DC offset and I/Q mismatches (I/Q amplitude mismatch and I/Q phase mismatch).
Here, the I/Q amplitude mismatch refers to a mismatch in amplitude of I/Q-components of an output signal from a quadrature modulator. As factors which cause the I/Q amplitude mismatch, the following three factors may be contemplated: (1) a mismatch in amplitude between I/Q-components of a local signal applied to the quadrature modulator; (2) a mismatch in amplitude between I/Q-components of a transmission baseband signal applied to the quadrature modulator; and (3) a mismatch in gain between a path associated with the I-component and a path associated with the Q-component within the quadrature modulator.
The I/Q phase mismatch, in turn, refers to a deviation of a phase difference from 90 degrees between I/Q-components of an output signal from a quadrature modulator. As factors which cause the I/Q phase mismatch, the following three factors may be contemplated: (1) a deviation of a phase difference from 90 degrees between I/Q-components of a local signal applied to the quadrature modulator; (2) a deviation of a phase difference from 90 degrees between I/Q-components of a transmission baseband signal applied to the quadrature modulator; and (3) a deviation of the phase on paths associated with the I-component and Q-component within the quadrature modulato.
A number of signal processing circuits have been proposed in the past with means for compensating a quadrature modulator for a DC offset and I/Q mismatches, but ideally, hardware added for compensation is closest possible to zero. In this respect, conventional signal processing circuits still have room for improvements.
FIG. 1 is a diagram showing an exemplary configuration of a conventional signal processing circuit.
Referring to FIG. 1, the signal processing circuit of this conventional example comprises transmission BB (baseband) signal generating part 11; test signal generating part 12; switch 13 labeled SW; DC offset and I/Q mismatch compensating part 14; DA converter 15 labeled DAC; quadrature modulator 16; envelope detector 17; AD converter 18 labeled ADC; and compensation amount generating part 19.
Transmission BB signal generating part 11 generates a transmission baseband signal in a transmission operation.
Test signal generating part 12 generates a test signal in a compensation operation.
Switch 13 selects the test signal generated by test signal generating part 12 in a compensation operation, and selects the transmission baseband signal generated by transmission BB signal generating part 11 in a transmission operation.
DC offset and I/Q mismatch compensating part 14 corrects a signal selected by switch 13 based on the compensation amount which has been set by compensation amount generating part 19. In this regard, the compensation amount is set in a compensation operation.
DA converter 15 D/A converts I/Q-components of the signal corrected by DC offset and I/Q mismatch compensating part 14 from digital signals to analog signals.
Quadrature modulator 16 mixes I/Q-components of the signal D/A converted by DA converter 15 with I/Q-components of a local signal, respectively for up-conversion, and adds I/Q-components of the up-converted signals to each other. An RF signal, which has been quadrature modulated in this way, is output from the signal processing circuit.
Envelop detector 17 detects the amplitude of an envelope (envelope) of an output signal of quadrature modulator 16.
AD converter 18 ND converts the output signal of envelope detector 17 from an analog signal to a digital signal.
Compensation amount generating part 19 generates a compensation amount for compensating quadrature modulator 16 for a DC offset and I/Q mismatches, based on the digital signal ND converted by AD converter 18, and sets the compensation amount in DC offset and I/Q mismatch compensating part 14.
In the following, a description will be given of the operation of the signal processing circuit of this conventional example.
In a compensation operation, switch 13 selects a test signal generated by test signal generating part 12. This test signal is applied to a baseband port of quadrature modulator 16 through DC offset and I/Q mismatch compensating part 14 and DA converter 15 for performing quadrature modulation. Envelope detector 17 detects the amplitude of the thus quadrature modulated signal, and AD converter 18 converts the detected amplitude to a digital signal. Compensation amount generating part 19 generates a compensation amount based on this digital signal.
FIG. 2 is a diagram showing a typical test signal used to compensate quadrature modulator 16. The test signal typically has an I-component which is a cos wave and a Q-component which is a sine wave.
FIG. 3 is a diagram showing the constellation of an output signal from quadrature modulator 16 which is in an ideal state. An ideal state for quadrature modulator 16 refers to a state in which either the I-component or Q-component does not suffer from a DC offset, and quadrature modulator 16 does not suffer from either an I/Q amplitude mismatch or an I/Q phase mismatch. In this event, the constellation draws a true circle centered at the origin. Accordingly, quadrature modulator 16 generates an output signal, the envelope of which is a sine wave of a regular envelope.
FIG. 4 shows the constellation of an output signal from quadrature modulator 16 which presents a DC offset. In this event, the constellation draws a circle, the center of which shifts from the origin. Accordingly, the envelop of the output signal from quadrature modulator 16 increases and decreases over time.
FIG. 5 is a diagram showing the constellation of an output signal from quadrature modulator 16 which presents an I/Q amplitude mismatch, and FIG. 6 is a diagram showing the constellation of an output signal from quadrature modulator 16 which presents an I/Q phase mismatch. In either case, the envelope of the output signal from quadrature modulator 16 increases and decreases over time.
Compensation amount generating part 19 matches the cycle and phase of the output signal from quadrature modulator 16, the envelope of which increases and decreases, with the phase and frequency of the test signal to confirm how much a DC offset, an I/Q amplitude mismatch, and an I/Q phase mismatch exist in quadrature modulator 16, and generates a compensation amount which is set in DC offset and I/Q mismatch compensating part 14.
On the other hand, in a transmission operation, switch 13 selects a transmission baseband signal generated by transmission BB signal generating part 11. This transmission baseband signal is applied to DC offset and I/Q mismatch compensating part 14, and is corrected on the basis of the previously set compensation amount. The corrected signal is applied to a baseband port of quadrature modulator 16 through DA converter 15 to perform a quadrature modulation. The signal, which has been quadrature modulated in this way, is an output signal of the signal processing circuit.
As a document which discloses a technique similar to the foregoing, there is Patent Document 1. This Patent Document 1 discloses a method of compensating a quadrature modulator for an I/Q phase mismatch using a test signal which comprises only two points in the first quadrant to the fourth quadrant on the I/Q quadrature coordinates. According to this method, the test signal is simplified.
As another document which discloses a technique similar to the foregoing, there is Patent Document 2. Patent Document 2 discloses a method of performing a compensation operation with a sinusoidal test signal applied to a baseband port of a quadrature modulator.
As another document which discloses a technique similar to the foregoing, there is Patent Document 3. Patent Document 3 discloses a method of improving the accuracy of a quadrature modulator based on a signal which is generated by frequency converting transmission data by an I/Q quadrature down converter.
As well, Patent Documents 4˜9 are presented as documents which disclose techniques similar to the foregoing.
However, the foregoing related techniques imply problems described below.
The related techniques described in conjunction with FIGS. 1 through 6 have a problem in that a large ROM area is required to store waveform data of a test signal because the test signal is a smooth sinusoidal wave. They also has a problem in that an AD converter is required to A/D convert the output of an envelope detector.
In the related technique disclosed in Patent Document 1, since two points on the I/Q quadrature coordinates are used for a test signal, simplification of the test signal is implemented. However, a problem is that an AD converter is required to A/D convert the output of the envelope detector. Also, Patent Document 1 discloses only a method of compensating for an I/Q phase mismatch, but does not disclose a method of compensating for an I/Q amplitude mismatch. Also, to perform a compensation operation for I/Q phase mismatch using the method disclosed in Patent Document 1, it is necessary to suppress a DC offset in terms of a baseband input of the quadrature modulator, i.e., a carrier leak in terms of the output of the quadrature modulator to be sufficiently low, and to suppress an I/Q amplitude mismatch to be sufficiently low.
Here, the method disclosed in Patent Document 1 will be described in greater detail with reference to FIGS. 7 and 8. Consider herein an example in which the quadrature modulator is compensated for the I/Q phase mismatch using a test signal which comprises two points in the first quadrant and second quadrant on the I/Q quadrature coordinates.
FIG. 7 is a diagram showing an example of the test signal at two points for use in a compensation operation for the quadrature modulator. Assume herein that the quadrature modulator does not suffer at all from either a DC offset, or an I/Q phase mismatch, or an I/Q amplitude mismatch. In FIG. 7 point 5 is a point in the first quadrant, and point 6 is a point in the second quadrant.
According to the method disclosed in Patent Document 1, the distance from the origin to point 5 and the distance from the origin to point 6 are found by detecting the amplitude of the envelope of the output signal from the quadrature modulator, and the condition under which these distances become equal is found as a condition under which no I/Q phase mismatch exists. In the example shown in FIG. 7, obviously, the method disclosed in Patent Document 1 correctly functions.
However, a DC offset in terms of an input exists without exception in the quadrature modulator due to a problem of its manufacturing accuracy of the quadrature modulator.
FIG. 8 is a diagram showing an example of the test signal at two points for use in a compensation operation for the quadrature modulator when a DC offset exists in the quadrature modulator. Again, assume herein that the quadrature modulator does not suffer at all from either an I/Q phase mismatch or an I/Q amplitude mismatch.
The example of FIG. 8 differs from the example of FIG. 7 in that a positive DC offset occurs in an I-component, and a negative DC offset occurs in a Q-component. Consequently, even if there is no input to the quadrature modulator, a comparable carrier leak occurs at point 0′. Due to this DC offset, point 5 and point 6 for use in a compensation operation shift to point 5′ and point 6′, respectively, in a lower right direction.
According to the method disclosed in Patent Document 1, the distance from the origin to point 5′ and the distance from the origin to point 6′ are found by detecting the envelop of the output signal from the quadrature detector, and the condition under which these distances become equal is found as the condition under which no I/Q phase mismatch exists.
However, according to the example of FIG. 8, the distance from the origin to point 5′ is obviously longer than the distance from the origin to point 6′ in spite of the absence of I/Q phase mismatch. In other words, the method disclosed in Patent Document 1 does not function correctly when a DC offset exists in the quadrature modulator.
Also, even supposing that a condition is found in the example of FIG. 7 under which the distance from the origin to point 5 is equal to the distance from the origin to point 6, the I/Q amplitude mismatch amount must be known in order to find the I/Q phase mismatch amount as an angle or its sinusoidal function value based on the amplitudes of the I-component and Q-component of the test signal at that time. In other words, in order to know two unknown amounts which are the I/Q amplitude mismatch amount and I/Q phase mismatch amount, another equation is required due to insufficiency of a single equation which represents the distance from the origin to point 5 that is equal to the distance from the origin to point 6. Actually, the equation disclosed in Patent Document 1 does not take into consideration the influence of the I/Q amplitude mismatch.
Assume herein that when two points (I,Q)=(1.05, 1.00) and (−0.95, 1.00) are applied to the quadrature modulator as the test signal, outputs signals of the quadrature modulator are equal in signal strength. Assume also a situation where an I/Q amplitude mismatch is present to cause the amplitude of the I-component to be k-times higher than the amplitude of the Q-component. In this event, the following Equation 1 is established when an I/Q phase mismatch amount is represented by X:(1.00 cos X)2+(1.05k+1.00 sin X)2=(1.00 cos X)2+(0.95k+1.00 sin X)2  [Equation 1]
Thus, it is obviously understood that I/Q phase mismatch amount X cannot be found unless k is previously known.
As a method of avoiding this problem, there is a method which is thought to remove the DC offset and I/Q amplitude mismatch of the quadrature modulator using another conventional technique before executing the method disclosed in Patent Document 1.
In reality, however, it is common to employ a DA converter for compensating for a DC offset. Nevertheless, when a DA converter is employed, a certain amount of DC offset inevitably remains due to the problem of resolution of the DA converter. In addition, relatively large values, i.e., −15 dB ˜−20 dB, are allowed for the amount of carrier leak that is allowed for a normal radio communication system, i.e., the value of a DC offset in terms of the baseband input of the quadrature modulator, with respect to a total transmission power. Accordingly, to use the method disclosed in Patent Document 1, the DC offset must be removed at an accuracy significantly more strict as compared with the amount of DC offset allowed for the radio communication system, resulting in a more complicated implementation.
Further, in a transmitter which employs a modulation scheme which does not use frequency near DC such as an OFDM system, an AC coupling is often used for coupling of a quadrature modulator with a DA converter in order to reduce a DC offset and low frequency component noise. When AC coupling exists, signals deteriorate because many DC components are included in the waveform of the test signal in the method disclosed in Patent Document 1. As a result, the method disclosed in Patent Document 1 does not correctly function.
The conventional technique disclosed in Patent Document 2 has a problem in that the configuration is more complicated, such as the need for a test signal generating part for generating a smooth sinusoidal test signal and an AD converter.
In addition to the problem in which the configuration is more complicated, the conventional technique disclosed in Patent Document 3 has a problem in that the I/Q accuracy and the like of an I/Q quadrature down converter for use in a compensation operation constitute causes for errors in the compensation operation. More specifically, to perform a compensation operation with high accuracy, an I/Q quadrature down converter is required to exhibit high I/Q accuracy. However, it can be said to be contradictory that while the compensation operation is performed due to the inability to manufacture a quadrature modulator having high modulation accuracy, an I/Q quadrature down converter having a high I/Q accuracy is required as a means therefor.
The conventional techniques disclosed in Patent Documents 4, 5 also have a problem in that the configuration is more complicated, such as the need for a test signal generating part, an AD converter and the like, as mentioned above, but Patent Documents 4, 5 do not disclose means for solving this problem.
In the conventional technique disclosed in Patent Document 6, four points are found on a phase plane, at which output signals from a quadrature modulator are equal in output intensities, to simultaneously find compensation amounts for an I/Q phase mismatch and a DC offset. However, even with this technique, an AD converter is required. It is also necessary to calculate the compensation amounts by solving quadruple simultaneous equations with 12 variables, described as Equation (6) in Patent Document 6, from data derived within the range of a limited resolution of an AD converter. Consequently, there is a problem in which hardware must be added in order to solve the quadruple simultaneous equations, and a problem in which the accuracy of the compensation amount exacerbates. Also, no solution is disclosed in Patent Document 6 for a problem in which an I/Q amplitude mismatch amount must be previously found.
In the conventional technique disclosed in Patent Document 7, the compensation amount for an I/Q amplitude mismatch is found from three pieces of information, the signal strength of each input signal of I/Q-components of a quadrature modulator, and the signal strength of an output signal from the quadrature modulator. However, this technique also requires an AD converter. Also, the influence of a DC offset and an I/Q phase mismatch that are exerted on the signal strength of the output signal from the quadrature modulator is not taken into consideration. For this reason, this technique correctly operates only when neither the DC offset nor I/Q amplitude mismatch exists.
In the conventional techniques disclosed in Patent Documents 8, 9, since a quadrature demodulator is used as a detection system in a compensation operation for I/Q phase mismatch and I/Q amplitude mismatch, a problem arises in which the accuracy of the quadrature demodulator can cause errors in the compensation operation. In other words, a quadrature demodulator having high I/Q accuracy and the like is required to perform a highly accurate compensation operation. However, it can be said to be contradictory that while the compensation operation is performed due to the inability to manufacture a quadrature modulator having a high modulation accuracy, a quadrature demodulator having high I/Q accuracy is required as a means therefor. Also, if a DC offset remains in the quadrature demodulator in the compensation operation for I/Q phase mismatch, this DC offset is reflected as an offset of a compensation amount for correcting a transmission baseband signal, thus leading to an increase in carrier leakage of the quadrature modulator. While a configuration may be contemplated to comprise means for compensating the quadrature demodulator to perform a compensation operation for the quadrature modulator after a compensation operation has been performed for the quadrature demodulator, Patent Documents 8, 9 do not disclose means for compensating the quadrature demodulator.
Also, in the conventional techniques disclosed in Patent Documents 8, 9, from the fact that a quadrature demodulator is employed as a detection system in a compensation operation, after output signals of I/Q-components from the quadrature demodulator have been converted to digital data by an AD converter, operational processing is performed to find an I/Q phase mismatch. Specifically, the compensation amount for an I/Q phase mismatch must be found from data which is made available within the range of a limited resolution of the AD converter, resulting in a problem of exacerbation in the accuracy of the resulting compensation amount. Patent Documents 8, 9 do not disclose a solution for this problem.
Patent Document 1:JP-2002-252663-APatent Document 2:JP-08-213846-APatent Document 3:JP-09-504673-APatent Document 4:JP-2004-007083-APatent Document 5:JP-2004-509555-APatent Document 6:International Publication No.2003/101061, PamphletPatent Document 7:JP-06-350658-APatent Document 8:JP-2004-274288-APatent Document 9:JP-2004-363757-A